The present invention relates to a double helix comprised of two partial helices of like winding direction, said partial helices each having an oval cross-section, substantially straight upper and lower winding legs and oppositely disposed winding arcs with mutually facing winding arcs of the two partial helices encircling each other. The invention further relates to the formation of the double helix by simultaneous wrapping two synthetic resin wires about a flattened mandrel and a spiral link belt fabricated from a plurality of said double helix structures.
A double helix is disclosed in EP-A-116 894 in which the partial helices are symmetrical in shape, i.e., the upper winding legs extend with respect to the longitudinal axis of the partial helices at the same angle as the lower winding legs. Such double helices are difficult to handle because the two partial helices have the tendency to slip one into the other. For the manufacture of a spiral link belt it is necessary, however, to laterally offset the partial helices relative to one another so that one partial helix can be joined to a partial helix of the next double helix by means of a pintle wire. Therefore, it is necessary to secure the two partial helices of one double helix in the laterally offset position, e.g., by applying an adhesive tape.
DE-A-30 47 989 discloses a simplified assembly of individual helices to form a spiral link belt wherein the individual helices have one set of winding legs, e.g., the lower winding legs, extending at right angles to the longitudinal axis and the other winding legs, in this case the upper winding legs, crimped so that they alone substantially determine the pitch of the helices. In this way, two individual helices of like or opposed winding direction can be shifted one into the other only somewhat less than half their width. In order to have a sufficiently wide channel for insertion of the pintle wire, the helices must be formed such that they have especially long winding legs. This is unfavorable if spiral link belts of low air permeability are to be produced.
Asymmetrically wound individual helices have also been disclosed in DE-A-30 01 472. The special configuration of the helices is apparent only from the drawing and it is not discernible whether it results in a simplification in the assembly of the individual helices to form a spiral link belt.
DE-A-30 03 344 discloses a method for the simultaneous formation of two individual helices for a spiral link belt in which two synthetic resin wires are wound by means of revolving thread guides in like sense on a stationary winding mandrel so as to cross each other alternately in each winding. The thus produced helices can be laterally separated after wind-up and a pintle wire inserted through the axis of the winding mandrel into the two individual helices to hold them together. The two individual helices connected by the pintle wire are then pulled apart against the pintle wire or turned apart around the pintle wire and are then assembled with further individual helix pairs to form a spiral link belt. The winding arcs of the two individual helices formed by this method and then interconnected do not embrace each other but are held together exclusively by the inserted pintle wire. Since the thus formed pairs of individual spirals have the same sense of winding, the contacting winding arcs of adjacent helices are not arranged in parallel but are arranged at a double pitch angle to the opposite alignment of the in reengaging windings, such pairs of individual helices develop a high degree of twist and can virtually not be processed. Moreover, this increases the spacing of the turns, i.e., the pitch of the spirals, excessively so that the clamping action is missing which facilitates the assembly of the helices.
A further difficulty in this mode of operation for winding helices results from the fact that the winding legs have different lengths. The winding legs extend at different angles relative to the longitudinal axis of the helices and the winding leg extending at the wider angle additionally travels along a longer path due to the fact that is crosses a winding leg of the other helix. During thermosetting of the spiral link belt assembled therefrom, this difference in length has the consequence that the winding arc is pulled somewhat around the pintle wire. This results in substantial deformation and damage to the pintle wire and to the helix wire. The individual helices produced pair-wise according to the method of DE-A-30-03-344 with an oval cross-section have an excessively high pitch. In the production of helices having an oval cross-section by winding the wire onto a mandrel, the helix wire automatically widens in the region of the winding arcs because at this point, the tension in the fed helix wire increases briefly due to the greater transverse dimension of the mandrel. These flattened regions are, in fact, desirable as coupling heads. However, when two helix wires are wound onto a mandrel, this has the consequence that the helices formed in pairs have a pitch corresponding to twice the width of the coupling heads. Several methods for assembling helices to form a spiral link belt, however, require tension spring-like bias within the helices which in turn is possible only if the helices have a pitch not higher than twice the helix wire diameter.